The ISO says emission controls on power plants have helped. But it says the decline is mainly due to increased use of natural gas-fired generators, which are more efficient and have fewer emissions than oil- and coal-fired generators.

As of 2011, natural gas accounted for 52 percent of the region's energy production, up from 15 percent in 2000. Oil accounted for less than 1 percent of energy production, down from 22 percent in 2000. Coal has fallen from 18 percent to 6 percent during that time.